Technical Abstract:
Test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell testing during 2010 were examined to assess the status of national milk quality. Somatic cell score (SCS) is reported to AIPL and was converted to somatic cell count (SCC) for calculating herd and State averages. The current legal limit for bulk tank SCC is 750,000 cells/ml for Grade A producers, but it has been proposed to reduce it by steps to 400,000 cell/ml by January 1, 2014. Nationally, average SCC during 2010 was 228,000 cells/ml. The percentages of herd test days that exceeded 750,000, 600,000, 500,000, and 400,000 cells/ml during year 2010 were 2.7, 5.7, 10.0, and 18.0, respectively. Thus, 2.7 of test-days from DHI herds were higher than the present legal limit for SCC of bulk tank milk. This may overestimate the percentage of herds that would have exceeded the legal limit for bulk tank SCC on test day because the milk of cows treated for mastitis is excluded from the bulk tank but not excluded from the DHI test. Also, the percentage of herd test days exceeding the legal limit would have been higher than the percentage of herds that were rejected from the market because market exclusion only occurs after repeat violations. Variation among States was large. Average SCC's were generally lower in the Mountain and Western States and often higher in the Southeastern States. SCC differences between adjacent states were substantial, which suggests that mastitis-control regimens have an impact under similar climatic conditions. As herd size increased, average daily milk generally increased, while average SCC declined. Notably, herds with <50 cows had 9.6% of test days over 600,000 cells/ml while herds with over 1,000 cows had no test days over this limit. Herd test day averages by month showed that milk yield decline during the summer months. SCC increased from May through August, then declined in September through November. The highest quality milk was produced in November and December.